Diels-Alder reaction

A Diels-Alder reaction depicted using molecular orbitals.
See main text for description and key

A Diels-Alder reaction in which a diene (butadiene, left) adds on to a dienophile (1,4-benzoquinone, center),
yielding a Diels-Alder product (right)

Also known as diene synthesis, a reaction
discovered by Otto Diels and Kurt Alder,
which is important in many chemical syntheses, including those involved
in making some plastics, insecticides, and
fungicides.

A conjugated diene, such as 1,3-butadiene isoprene,
adds readily to a "dienophile" containing a double or triple bonded activated
by an adjacent nucleophile group.

The Diels-Alder reaction has served an important role in many syntheses
because it smoothly and stereospecifically unites two carbon compounds.
A six-membered ring is formed by 1,4-addition of an alkene unit to a conjugated diene. The alkene unit (known as the dienophile) has
electron-withdrawing groups which activate the diene.

The upper illustration here shows a Diels-Alder reaction using molecular
orbitals. The addition reaction is shown by the rearrangement of the electron
clouds.

Key (to upper illustration)

A) Butadiene. This is a conjugated diene (two double bonds separated by
a single bond). It is shown is the s-cis form and has the property of undergoing
1.4 addition very easily.

B) 1,2-dintrile ethene. This is the dieneophile, and the electron-withdrawing
or accelerating groups are nitrile groups.
It is these that activate diene.

C) The diene and dienophile react to form 2,3-dintrile benzene. The configurations
of the diene and dienophile are retained in the product, which means that
the reactants come together to give cis addition.